A material for a membrane for water treatment should have a high hydrophilicity, chlorine resistance, and tensile strength. More specifically, as the hydrophilicity increases the membrane is less likely to be contaminated; as the chlorine resistance increases a contaminated membrane can be washed without being damaged; and as the tensile strength increases the membrane can better tolerate a pressure during a long term use.
Conventionally, cellulose acetate, which has low tensile strength and chlorine resistance but high hydrophilicity, thus reducing membrane contamination and extending a membrane washing cycle, and polyvinylidene difluoride (PVDF), which has low hydrophilicity but high chlorine resistance, thus being suitable for washing, have been mainly used as materials for a membrane for water treatment.
Cellulose acetate as a membrane material for water treatment is advantageous in that it can minimize membrane contamination due to its high hydrophilicity but it has a disadvantage of having a low mechanical strength. That is, when cellulose acetate is prepared a polar catalyst such as inorganic acid must be used because the synthesis of cellulose acetate requires acetylation, which destroys the crystalline structure of cellulose used as a raw material. As a result, the cellulose backbone is easily broken and the finally produced cellulose acetate has a low molecular weight and an extremely low mechanical strength.
In contrast, polyvinylidene difluoride, which has excellent mechanical strength and chlorine resistance but low hydrophilicity, is disadvantageous in that a membrane prepared using the same is easily contaminated thus requiring frequent washing.